A micromechanical structure, such as, for example, a displaceable micromirror, has a fixing point, a silicon spring, and a movable part. The silicon spring is connected to the fixing point at a first end and to the movable part at a second end. Situated on the silicon spring is at least one copper circuit trace extending at least from the first end to the second end.
MEMS sensors or actuators are often made up of a fixed element (referred to in the following as “mainland”) and a movable element (“island”), which are joined to each other by a flexible structure (“spring”). If an electric current is needed on the island, then this must be brought there via the spring. In certain applications, the spring must now be narrow in order to satisfy specific mechanical requirements, but at the same time, a high electric current must be carried by the spring, which means that a material having a low specific resistance is needed as a current-carrying conductor. Osaka et al., “Electrochemical Deposition Process for ULSI Interconnection Devices,” in “Modern Electroplating,” edited by Mordechay Schlesinger and Milan Paunovic, Chapter 13, describes a method in which pure copper is used for a current-carrying conductor, the copper being deposited electrochemically and patterned to form circuit traces, using the Damascene process. This copper is already subject to an annealing process (self-annealing) at room temperature, through which the grain size increases into the range of several μm. This grain size is already in the range of typical layer thicknesses and circuit-trace widths used.
The function of the spring is to deform mechanically in order, thus, to allow movement of the island with respect to the mainland. This deformation must be purely elastic, in order to be reversible and reproducible. In addition, in the case of a purely elastic system, no material aging or material fatigue takes place. Furthermore, in the case of a spring deformed purely elastically, the tension at its end connected to the mainland is directly proportional to the displacement of the island. Thus, this displacement can be determined reliably, using, for example, piezoresistive bridges at the connecting point between the mainland and the spring. Silicon is mostly used as a material for the spring based on favorable mechanical properties, in particular, purely elastic behavior up to the failure limit.
Pure metals, which have a high conductivity, are very soft, that is, they have a low yield point, and thus, also the above-described, Damascene copper (as well as Au, Al). If they are used as a conductor on an above-described spring, then, in response to mechanical deformation of the spring, the film tension in the conductive material generally becomes so high, that the conductive material deforms plastically. Due to that, the overall element of the spring no longer behaves purely elastically, and the advantages described above are lost. In particular, it is no longer possible to highly accurately determine the position of the structures suspended on the spring.
In order to eliminate this conflict, according one solution, the spring is designed so that the tension in the conductive material always remains below the (very low) yield point of the conductive material. However, this is not possible for many applications. Alternatively, use of alloys in place of pure materials as conductive materials is possible. In this instance, the yield point of the metal is greater than the maximum mechanical stress occurring. In a semiconductor process, such as in the case of MEMS, this is technically highly difficult to carry out and, in the case of copper, also accompanied by a reduction in the electrical conductivity.
U.S. Pat. No. 8,218,218 describes a solution, which uses different conductive materials separated by insulating layers, in a multilayer system, which then have to be reconnected to each other at the ends of the spring. This is a technically highly complex solution, in which, additionally, on the basis of the height of the overall structure, electrical conductivity is markedly lost due to the insulating layers used. Alternatively, it is possible to increase the yield point by reducing the geometric dimensions of the metallic traces. However, the result of this is that the available conductive cross section and, consequently, the maximum current are markedly reduced.